368 ROBERT J. TERRY 



the otic region. There remains, however, a wide gap, equal in 

 extent to the dorsal margin of the ear capsule, between this 

 tract of cartilage, which I have referred to as the beginning of 

 the commissura orbitoparietalis, and the parietal plate. Now in 

 the mole, Noordenbos found a cartilaginous plate immediately 

 anterior to the tectum interoccipitale and identified it as the 

 parietal plate. It united subsequently with the otic capsule, 

 opposite the junction of the anterior and middle thirds of the 

 anterior semicircular canal (synchondrosis parieto-canalicularis) , 

 with the anterior margin and ventral end of the tectum interoc- 

 cipitale, and also with the ala orbitalis. The last connection 

 came about through the synchondrosis orbito-parietalis, by ex- 

 tension of the lateral hinder angle of the ala orbitahs. 



In sections of cat embryos of 17 mm. a parietal plate of young 

 cartilage extends from the occipital arch to the commissura or- 

 bito-parietalis. The arch of the tectum posterius is represented 

 merely by a prong, springing on either side from the broad plate 

 of cartilage forming the caudal end of the parietal plate, and 

 which is united by stretches of young cartilage with the pars 

 canalicularis and lateral occipital arch. The anterior end of the 

 parietal plate, while approaching the otic capsule at the level of 

 the middle of the anterior semicircular canal, is still not united 

 with it, but is separated by a plane of mesenchyma. In later 

 stages (24 to 30 mm.) van Wijhe preparations show that chon- 

 drification is less advanced, in the narrow part of the parietal 

 plate, where the inconstant union with the pars canalicularis 

 occurs (p. 310), and where union has occurred with the occipital 

 arch and otic capsule. There seems to be no doubt of the car- 

 tilaginous plates, identified as the beginnings of the parietal plate, 

 going to form also the tectum posterius (the arch is complete in 

 the 23.1 mm. stage) and also the posterior part of the parietal 

 plate. The relation of the conamissura orbito-parietalis to the 

 otic capsule anteriorly (the so-called parieto-capsular commissure) 

 is evidence of the probable homology of this cartilage with the 

 parietal plate of Talpa, as described by Noordenbos. There is 

 no evidence of another center of chondrification between the ter- 

 mination of the orbito-parietal commissure and the tectum pos- 



